This thematic set brings together invited papers given at the Lyell Meeting on 14 February 1991. Despite the Siberian weather, over 60 scientists descended on Burlington House from 12 different countries to attend this meeting, which brought together a wide range of specialists. A major aim of the meeting was to discuss the background to the ‘Cambrian Explosion’ by focussing on various aspects of ‘Geological and biological changes across the Proterozoic-Phanerozoic transition’.

This interval (Fig. 1) marked a major turning point in the geological record, during which the major invertebrate groups appeared. Rapid evolutionary diversification (Fig. 2) is clear from data presented in the following papers. Decline in the abundance and diversity of the soft-bodied Ediacara fauna during the Kotlinian Crisis was followed by an adaptive radiation that included trace fossils, acritarch phytoplankton and a variety of early skeletal fossils. The rise and subsequent fall of the early Cambrian biota is most dramatically picked out by the history of archaeocyathan sponges (Fig. 2) which largely became extinct during the Toyonian Crisis.

No biostratigraphical framework existed for this remote part of the scale prior to 1960. J. W. Cowie gives the background to the last two decades of steady progress by members of IGCP Project 29 on the Precambrian-Cambrian boundary, leading up to a vote on the boundary stratotypes at about the time of the Lyell Meeting. In the interval between the meeting and going to press, a majority vote has favoured placing the Precambrian-Cambrian boundary at a point marked by

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